The term gray iron is generic rather than specific and covers a wide range of alloys of iron, carbon and silicon. In usual commercial practice these alloys range from somewhat above 2.2 per cent carbon up to about 3.8 per cent, with silicon content 0.5 to 3.0 per cent. Carbon in excess of the amount contained in the combined form (that is, in the pearlite present) occurs in the form of free graphite flakes—hence, gray iron is free from white chill spots due to massive cementite. The physical properties at room temperature of various gray irons vary considerably. The tensile strength of different classes or grades runs from about 18,000 up to 60,000 lb. per sq. in., the Brinell hardness ranges from 120 to 240, and many other properties vary over wide ranges, according to the class or grade, and according to the section size of the casting.